GROUND STATION USER EQUIPMENT for ANDE WB4APR
------------------------------------------------------------------
13 Nov 2002
We did some DATA performance tests on two D700 mobile radios as a
followup to prior testing we had done on the Handheld D7. The
results were very similar with 1200 baud being about 7 dB better
than 9600. Thus, we conclude that for reliable delivery of short
text messaging to mobile and handheld satellite users, that a 1200
baud 2m downlink is 7 dB stronger and the most effective from a power
budget standpoint. Going to 9600 baud requires 5 times more power
and only gains about a doubling of throughput due to TX delays...
TEST CONDITIONS: Two D700's were separated by about 1/4 mile with
30 to 40 dB of attenuators on both ends connected to mobile whips.
Radios were operated at the lowest power setting. Attenuators were
adjusted to find the 50% and 100% performance rate at 1200 and 9600
baud on the Smeter. Then we used a HP Sig gen to calibrate these
readings to dBm. S meter readings shown are the number of LCD
segments lit.
BAND FREQ DATA RATE 50% 100% Comments
---- ------- --------- ------- ---------- ----------------
A 145 MHz 1200 baud S0 -123dBm S0 -120dBm
B 145 MHz 1200 Baud S0 -121dBm S0 -118dBm About 2 dB worse
A 145 MHz 9600 baud S2 -116dBm S3 -113dBm About 7 dB worse
B 435 MHz 1200 baud S0 -122dBm S0 -119dBm
B 435 MHz 9600 baud S3 -115dBm S3 -111dBm About 7 dB worse
PACSATS: The above tests are between two identical radios. Our
PACSAT experience has shown that the 9600 baud PACSAT downlinks
often decode with even less signal, often at S0! So they do seem
to have optimized signals.
PRIOR TEST OF KENWOOD TH-D7 HANDHELD PERFORMANCE:
During July 2000, W4HFZ on temporary assignment to the Academy,
and I compared 1200 and 9600 baud performance using the SAME radios
under the same channel under controlled conditions. The results in
this test do show about a 7 dB difference between 1200 baud AFSK
and 9600 baud FSK.
TEST CONDITIONS: Two THD7 Handhelds were separated by 100 feet of
RG58 with 70 dB or more of SMA RF attenuators on both ends and with
RF clamp on ferrite chokes at 4 places along the coax. Radios were
operated at Extremely Low power setting.
PROCEDURE: Attenuators were adjusted to find the 50% and 100%
performance rate at 1200 baud, then we changed to 9600 baud and re-
adjusted attenuators to get the same performance. S signals are the
number of LCD segments lit.
BAND FREQ DATA RATE 50% 100% Comments
---- ------- --------- ------- ------- ----------------
A 145 MHz 1200 baud S1 -120dBm S1 -117dBm
A 145 MHz 9600 baud S5 -113dBm S5 -110dBm About 7 dB worse
B 435 MHz 1200 baud S0 -119dBm S0 -116dBm
B 435 MHz 9600 baud S3 -111dBm S3 -109dBm About 7 dB worse
S-METER: Although the above test shows that there is a difference
in the S meter on the two bands, the RF signals are about the same.
Within each S meter range, there appears to be about 3 dB and that
is the difference between solid and marginal decoding.
PACSATS: The above tests are between two identical radios. My
PACSAT experience has shown that the 9600 baud PACSAT downlinks
often decode with even less signal, often at S0! So they do seem
to have optimized signals.
CAVEATS: Even though the radios are identical, it is still an
apple and oranges comparison, because, of course, the audio path
and filtereing is different between the two modems. But if we
"assume" that kenwood did the best they could with each, within
their other constraints, then this does give us a data point that
tends to confirm the performance difference between 1200 AFSK and
9600 FSK baud.
Given that there is a further 9 dB or so PATH loss difference
between VHF and UHF, this gives us a baseline for trying to design
the optimum simple 2-way digital messaging satellite links to hand-
helds. On the downlink, we want to deliver a message to an HT in
your pocket. On the uplink, we want to be able to transnmit to the
bird with the whip held in the clear.
Bob Bruninga, WB4APR